One thing I am thinking..the specialists cost a lot and I know it's been said but I personally think 15 regular towers > 1 specialist. Maybe add a skill to get extra gold from kills or lower the specialist by 100 or so?
Also, after reading a bunch of these comments. Yes it's hard to plan ahead for the most part but if you read the map description it kind of gives you an idea of what is going to be there. You can re-diss your skills and set up for that certain map. Also, go in and let a wave go by itself. You get a general idea of what is there..and don't be too heavy with the same type of class unless youve played through and know what you mean. Purple ones are just a good debuffer I only have 2 maybe 3 per map. Plus don't underestimate the poison mage, they do good damage with their DoT, spread them out.
Only real problem I've had and maybe I just read wrong but it seems like the Necromancers attack is absorbed by enemies with the electricity absorb skill. Is that supposed to be?